1. Field of the Invention
This invention relates to an image processing apparatus, and more particularly, to an apparatus capable of processing a color image, and an image reading apparatus.
2. Description of the Related Art
Heretofore, as an example of such image processing apparatus, there has been known an apparatus which converts an object image into electric signals using, for example, a scanner. As an example of such apparatus, there is known an apparatus which performs photoelectric conversion by means of a sensor having, for example, three light sources for R, G and B, and which switches emission colors by switching among the light sources.
In the above-described apparatus, however, if a method is used in which, for example, a luminance signal and a color signal are separately transmitted, or if each color component is separated and the color components are transmitted in parallel, for the transmitting of color images, there arises, first, the problem that the configuration at the reception side becomes complicated.
Recently, a tendency toward increasing personal use of business machines has been advancing, and remarkable technical progress toward smaller and cheaper machines is being made. Personal computers and word processors occupy the central core of these machines, and image scanners are utilized for preparing intercompany publicity, invitation cards and the like using personal computers or word processors.
Manual-scanning-type image scanners are mainly being used with such systems. With an image scanner, it is possible to perform a two-dimensional reading by forming an image of a manuscript illuminated by an LED array or the like on a line image sensor, and moving the main body of the scanner along the surface of the manuscript.
A handy-type black-and-white-image reading apparatus for inputting images is attached to the above-described personal computer or word processor. Color image processing also becomes possible keeping pace with the development of inks of colors, such as Y, M and C and the like. There has been thereby performed the development of reading apparatuses which convert color images into electric signals to be input to personal computers, word processors and the like. As such reading apparatus, a manual-scanning color scanner capable of performing color reading is desired. Although a scanner for monochrome manuscripts is sufficient for reading, for example, logo marks, demand for color reading is nevertheless strong for photographs and the like. This demand is becoming stronger as the tendency toward the greater use of personal and color displays printers advances.
As an image sensor for reading color images, there is known a CCD color image sensor which has plural photosensors aligned on a line and is provided with R, G and B striped filters in the direction of the line, and which performs color separation of the corresponding area on a manuscript by making continuous R, G and B signals for each line, or the like.
Color reading of a manuscript is usually performed according to the following process using, for example, the above-described CCD color image sensor.
First, the manuscript is subjected to color separation using red (R), green (G) and blue (B) filters, and converted into gradation information consisting typically of anywhere from 64 gradations to 256 gradations. Subsequently, various corrections, such as masking of the filters, .gamma.-correction, logarithmic transformation, ink correction and the like, are performed to obtain color image data for yellow (Y), magenta (M) and cyan (C).
However, when a manual-scanning color scanner is used, the manual-scanning color scanner must be light in weight and compact in size so that it can be promptly moved to a desired position.
Accordingly, there is a limitation on the processing circuit and the like which can be incorporated in the scanner in order to perform excellent color processing.
Furthermore, a manual-scanning scanner is moved at an arbitrary timing and an arbitrary speed. If the interval between reading scanning lines of a color image changes according to this arbitrary movement, distortion may occur in the output image.
In addition, the time for reading an image on an identical line differs when moving at a high speed and when moving at a low speed, and hence the density of an image fluctuates. There is therefore the second problem that, although the above-described problems can be corrected by simply changing the threshold value in a scanner for monochrome images having binary values, the problems can not be easily dealt with in a color scanner because the influence is not only on density but also on color balance.
Moreover, dynamic ranges of R, G and B signals read from a CCD color image sensor are in general not equal to one another. Accordingly, in an image reading apparatus in which analog color signals for three colors are amplified, the amplified signals are subjected to A/D conversion by an A/D converter, and image data are subjected to digital processing, the number of gradations from the white level to the black level of each image is different from one another for R, G and B.
This is because the spectral distribution of an illuminating light source in the visible region is not uniform, and there are variations in light transmittance of R, G and B filters attached to CCD color image sensors and the sensitivity as a function of wavelength of CCD color image sensors.
An example of the spectral distribution of an illuminating light source and an example of the sensitivity for wavelength of R, G and B picture elements are shown in FIGS. 23 and 24, respectively. The ordinates are measured in arbitrary (relative) units in both figures.
Relative to R, G and B signal levels when a white manuscript is illuminated by a light source having the spectral distribution shown in FIG. 23, the R signal is the largest and then there come the G and B signals, in descending order.
At this time, if a reference voltage of an A/D converter having a resolution of 5 bits is set for use with the output an R picture element, 32 gradations can be obtained for the R picture element, but only 26 and 20 gradations can be obtained for a G picture element and a B picture element, respectively.
When the number of gradations from the white level to the black level (more exactly, the maximum-saturation level) of each picture element for R, G and B thus differs, color separation and, consequently, image processing of a manuscript become difficult.
In order to solve this problem, the following method can be considered. That is, as shown in FIG. 25, analog-signal amplification units 421-423 for inputting picture-element signals of a CCD 401 are provided, and the gain of each unit is adjusted to each picture element for R, G and B. Outputs of the three amplification units 421-423 are sequentially selected by a selector 424 in synchronization with the driving timing of the CCD 401 to transmit to an A/D converter 403, and 32 gradations are obtained for all the three picture elements. However, this approach entails a third problem, that the circuit scale becomes large.
Now, a conventional line sensor has in general a configuration as shown in FIG. 26.
In FIG. 26, a photodiode array PD has plural photodiodes aligned in the form of a line. CCD analog shift registers CCD A and CCD B correspond to odd-numbered and even-numbered photodiodes, respectively, and transfer signal charges stored in each photodiode. An output stage O synthesizes signals transferred separately in the two channels by the CCD A and CCD B into one channel, and generates a video signal V.
In order to operate such a CCD color image sensor sequentially provided with R, G and B filters in the direction of the line, shift pulses SH, transfer pulses .phi..sub.1, and .phi..sub.2, and reset pulses RS are in general input to the image sensor with timings shown in FIG. 27.
The shift pulses SH are pulses for turning on and off shift electrodes. When the shift electrodes are turned on, signal charges in the photodiode array PD as a photosensitive unit are all transferred to the CCD shift registers CCD A and CCD B as transfer units. After the shift electrodes have been turned off, the signal charges are sequentially transferred through the CCD shift registers CCD A and CCD B by transfer pulses .phi..sub.1 and .phi..sub.2.
The CCD shift registers transfer signal charges one stage for every one period of the transfer pulses .phi..sub.1 and .phi..sub.2, and the transferred signal charges flow into a floating capacitor at the output stage O. These charges change the voltage of the floating capacitor by an amount V=q/c, where q is the net charges transferred to the capacitor and c is the capacitance. This change in voltage changes the current flowing through a load resistance, and is output as a signal OS of voltage.
Since it is necessary to return the voltage of the floating capacitor to an initial value for detecting the signal charges of the next picture element, reset pulses RS are added to the output stage O to cancel the voltage of the floating capacitor.
In the signal OS, the leading 48 bits are dummy bits, and an effective signal can be obtained from the 49th bit, in the example shown in FIG. 27. As the effective signal, each picture-element signal for R, G and B is sequentially output, a group of three consecutive picture elements, one each for R, G and B, are made one set, and color separation of an area on a manuscript corresponding to the one set is performed. The effective signal consists of 1560 bits, that is, 520 sets. After the entire effective signal has been output, additional dummy bits are output until the next SH pulse arrives.
Now, the signal processing procedure until the analog image signal OS output from the CCD line sensor illustrated in FIG. 26 is converted into a digital image signal will be explained with reference to FIG. 28.
Since a DC component is added to an image signal read by the CCD line sensor 501, the DC component is removed at a DC cut unit 601 within an amplification unit 502. The signal is then subjected to sampling-and-holding for every picture element by a sample-and-hold circuit 602, and the resultant signal is amplified by an amplification unit 603. Since a small amount of DC component as noise is added to the amplified signal, the DC component is removed by a DC cut unit 604. The amplified signal from which the noise has been removed is input to an A/D converter 503, and is subjected to analog-to-digital conversion for every picture element.
In the above-described conventional driving method, however, when there is adopted a method in which an image signal for one set of R, G and B picture elements is converted into a print signal for Y, M, C and Bk (Y, M, C and Bk indicate yellow, magenta, cyan and black, respectively) picture elements, it is necessary to output the signal for one set of Y, M, C and Bk within a time during which a signal for one set of R, G and B is read from the CCD.
At this time, if the .phi..sub.1 and .phi..sub.2 are made, for example, by frequency-dividing by 4 a system clock SCLK as a base clock, 12 clocks SCLK are required for reading a signal for three colors, R, G and B. Accordingly, when transmitting print data for 4 colors, Y, M, C and Bk, data for each color, Y, M, C and Bk, must be transmitted by 3 clocks, respectively.
Furthermore, if the .phi..sub.1 and .phi..sub.2 are made by frequency-dividing by 8 the system clock SCLK, print data for 4 colors are to be transmitted by 6 clocks for each color, Y, M, C and Bk, respectively, but clocks for one color become clocks multiplied by 3, such as 3, 6, 9, 12, - - - .
Now, when there is used a method in which, in an image reproducing unit, for example, the density corresponding to data consisting of one set of R, G and B to be input is reproduced as a visible image by the number of Y, M, C and Bk dots plotted within a 4.times.4 dot matrix, data for Y, M, C and Bk are output as print data consisting of 4 bits for each, that is, 16 bits in total, for the reading of an image signal consisting of one set of R, G and B.
At this time, when data are transferred by 6 clocks for each color, Y, M, C and Bk as described above, there can be considered a method in which 2 invalid bits are added to 4 bits of data for every 4 colors to made 6-bit data for one color, and the data are transferred by 6 clocks. However, since 33% of the data to be transferred is invalid, the transfer efficiency of data is inferior, and this method is therefore not preferable.
In order to solve this problem, it is necessary to provide a second oscillation circuit for data transfer separately from the system clock SCLK. Furthermore, the clock of the second oscillation circuit must be perfectly synchronized with the system clock SCLK.
In addition, when reading a white manuscript and the like, the output signal of the amplification unit 603 shown in FIG. 28 resembles a DC signal when there continues a state in which the magnitudes of R, G and B signals are uniform, as for a white manuscript. Hence, even a part of an image signal is removed at the DC cut unit 604.
Accordingly, there is the fourth problem, that it is difficult to obtain a high-quality color image.